1. Field of the Invention
This invention relates to the recovery of N-methyl-2-pyrrolidone (hereinafter referred to as NMP for the sake of brevity) employed in hydrocarbon extraction processes and prevents water buildup in the recovered solvent. More particularly, this invention relates to an improved process for removing minor amounts of water extraneously introduced into a lube oil extraction solvent comprising NMP and prevents water buildup in the solvent system. Still more particularly this invention relates to dehydrating said solvent by passing same, as a vapor and in combination with a non-aqueous stripping gas, to a rectification zone and condensing zone, thereby removing the water from the solvent without requiring any additional heat input into the solvent recovery system. 2. Description of the Prior Art
It is well known to use NMP as a solvent for extracting aromatic hydrocarbons from mixtures of aromatic and nonaromatic hydrocarbons. It is also well known in the art to use NMP as a lube oil extraction solvent wherein an extraction solvent comprising NMP is contacted with a lube oil fraction thereby extracting the undesirable aromatic and polar constituents from said fraction to produce extract and raffinate phases, the extract phase containing most of the solvent and undesirable lube oil constituents and the raffinate phase containing most of the lube oil.
The purpose of solvent refining lube oil fractions is to remove therefrom those constituents present therein that contribute to low viscosity index, poor thermal stability, poor oxidation stability and poor ultraviolet stability. These constituents are primarily aromatic and polar in nature. Other solvents well known in the prior art as being useful for lube oil extraction include, for example, phenol, phenol-water, furfural, sulfur dioxide, sulfur dioxide-benzyl, chlorex, etc., with the most common solvents being phenol-water and furfural. However, it has recently been found that NMP is somewhat superior to phenol and furfural as a lube oil extraction solvent in that it offers certain advantages such as increased yield of useful lube oils. Another advantage is that it does not form an azeotrope with water as do phenol and furfural, so that mixtures of water and NMP may be completely separated by simple distillation. However, one important disadvantage associated with the use of NMP is the fact that it is highly hygroscopic and absorbs water. This is important, because solvents used in hydrocarbon extraction processes are recovered and reused indefinitely. If water is allowed to build up in these solvents it changes their characteristics.
Adding water to NMP used in solvent extraction processes changes its characteristics in that as more and more water is added to the NMP its solvent power decreases and the solvent/oil miscibility temperature increases. The miscibility temperature is that temperature at which the solvent and oil become mutually soluble or miscible and only one liquid phase exists. In order to obtain the desired yield and quality of raffinate oil at a practicable extraction temperature, it is necessary to maintain the water content of the NMP within an appropriate range. Therefore, critical to the proper use of solvents comprising NMP for lube oil and other hydrocarbon extraction processes is the determination and maintenance of that amount of water that must be added to the solvent for each particular type of hydrocarbon feed. By way of example, when NMP is used to extract a relatively high VI paraffinic lube oil feedstock it preferably contains from 2-4 LV% (liquid volume) of water. As the paraffinicity of the feed decreases, the water content of the NMP can be increased up to as much as 10 LV% or more.
Whatever the optimum water content may be for a particular feedstock or operation, it is necessary to maintain that water content in order to achieve consistent and uniform extraction. However, even though no additional water is deliberately introduced into the solvent, it is possible for water to be accidentally introduced into the solvent and to build up to an undesirable level over a period of time. For example, oil feedstocks often absorb water from humid air while in tankage, steam coils used for heating oils and solvents containing NMP often develop minor leaks, etc. Therefore, in order to avoid changing the characteristics of the NMP-containing extraction solvent over a period of time due to the introduction and buildup of small quantities of extraneous water into the solvent inventory, the extraneously introduced water must be removed in order to maintain the water content at the desired level.
A number of complex solvent recovery schemes have been developed for recovering NMP in lube oil extraction processes. In U.S. Pat. No. 3,476,681 NMP is recovered from the raffinate phase by adding thereto a water-containing stream so as to effect separation of an NMP rich solvent from the raffinate (because NMP is more soluble in water than in oil), distilling and vacuum steam stripping residual NMP and water from the water-extracted oily raffinate phase, distilling the extract from the solvent extraction twice, followed by steam stripping, combining the distillate from both strippers to provide the water containing stream for removing (water extracting) the NMP from the raffinate and then finally separating the water from the NMP by distillation. U.S. Pat. No. 3,461,066 is directed towards a process for removing both NMP and extraneously introduced water from the extract phase of solvent extracted lube oil stocks via four consecutive distillations, resulting in essentially water-free NMP being recycled back to the extraction zone. Similarly, in U.S. Pat. Nos. 3,470,089 and 3,476,680 distillation is the method that is ultimately used for separating the recovered NMP from extraneously introduced water. However, in utilizing distillation for separating water from NMP a considerable amount of heat is required, because water has about five times the latent heat of evaporation as the NMP. Further, any distillation operation requires a heating and cooling cycle.
Therefore, it would be a considerable improvement to the art if a method could be found for removing minor amounts of extraneously introduced water from the NMP without the need for separate distillation units and the additional heating and cooling required to operate them.